In optical communication networks, optical amplifiers inserted in optical fiber transmission lines need to be set up in advance by setting their gain, which is an amplification factor determining the relationship of the output level with the input level.
The optical amplifier is set up by a method described below. When a system is introduced, for example, with no operational signal beam propagated, ASE (Amplified Spontaneous Emission) light with transmit power equivalent to that of a single-wavelength signal of the operational signal beam is emitted from the post-amplifier of a transmitting station, and the in-line amplifier of a downstream-side repeater station or the pre-amplifier of a downstream-side receiving station is set up by determining its gain with the use of the ASE light (in the following, the method using ASE light to set up an optical amplifier is referred to also as ASE setup).
In the case of an optical amplifier of which the principle of amplification is stimulated emission, such as an EDFA (Erbium-Doped Fiber Amplifier), the phenomenon of spontaneous emission takes place irrespective of the presence or absence of input light. Broadband noise light leaking from the optical amplifier due to this phenomenon is called ASE light.
FIG. 17 schematically illustrates the conventional ASE setup. An optical transmission system 5 comprises a transmitting station 51 and a receiving station 52 interconnected by an optical fiber transmission line f. The transmitting station 51 includes a post-amplifier 51a of which the output level of ASE light can be varied, and the receiving station 52 includes a DCF (Dispersion Compensating Fiber) 52a and a pre-amplifier 52b. 
A signal beam amplified by and output from the post-amplifier 51a is transmitted through the optical fiber transmission line f. The DCF 52a compensates the chromatic dispersion of the incoming signal beam caused on the optical fiber transmission line f. The pre-amplifier 52b receives and amplifies the dispersion-compensated signal beam.
When the ASE setup of the pre-amplifier 52b is performed prior to the start of operation of the optical transmission system 5, ASE light with transmit power equivalent to that of a single-wavelength signal of the operational signal beam is emitted from the post-amplifier 51a. 
The ASE light is propagated through the optical fiber transmission line f and reaches the receiving station 52, where the ASE light passes through the DCF 52a and then enters the pre-amplifier 52b. FIG. 17 also illustrates the power levels of the ASE light at respective points p1 to p3 (wherein the vertical axes indicate optical power and the horizontal axes indicate wavelength). The point p1 indicates the transmit power of the ASE light, and the point p2 indicates the receive power of the ASE light at the input end of the pre-amplifier 52b. The point p3 indicates the output power of the ASE light amplified by the pre-amplifier 52b. 
At the receiving station 52, the gain of the pre-amplifier 52b is determined by using the received ASE light such that the output power of the pre-amplifier 52b is equal to a target output value for an actual signal beam (target output value for the single-wavelength signal).
FIG. 18 illustrates the signal beam power during operation of the system after the ASE setup. More specifically, FIG. 18 depicts the power levels of an actual signal beam observed at the respective points p1 to p3 after the ASE setup illustrated in FIG. 17. Using the gain determined during the ASE setup, the pre-amplifier 52b amplifies the actual signal beam up to the target output level.
Thus, the ASE setup enables the gain of an optical amplifier to be determined efficiently prior to the start of operation of the system. Also, in a redundant configuration wherein in the event of a line fault, for example, the active system is instantly switched to the standby system, the optical amplifier may be kept ready (in a standby state) with its gain set to that determined by the setup, whereby the switchover time can be minimized.
As conventional techniques, a technique has been proposed in which automatic gain control is performed by using the ASE noise light separated from amplified light (Japanese Laid-Open Patent Publication No. 2001-144353 (paragraph no. [0013], FIG. 1)). A technique has also been proposed in which variable optical attenuators and tunable dispersion compensators are set and controlled so that the optical loss and the chromatic dispersion may be equal among multiple optical transmission lines, to allow the receiving-side amplifier to receive and amplify an optical signal whose input optical level is kept fixed (Japanese Patent No. 3833684 (page 5, lines 3 to 24, FIG. 1)).
In cases where light with multiple wavelengths is transmitted over a long distance, the chromatic dispersion and output level that vary from wavelength to wavelength need to be compensated at the repeater or receiving station. To this end, dispersion compensators such as DCFs or other compensation devices such as gain equalizers are inserted in optical fiber transmission lines.
In many conventional systems, fixed compensation devices (such as DCFs) with fixed dispersion compensation values or fixed equalization characteristics are used. In the future, it is expected that variable compensation devices whose compensation values are variably adjustable, such as a TDC (Tunable Dispersion Compensator) and a DGE (Dynamic Gain Equalizer), will be used more and more with a view to simplifying the system configuration as far as possible and cutting down costs by reducing the number of types of compensation devices to be kept in stock.
FIG. 19 illustrates the configuration of a DGE as an example of such variable compensation devices. The DGE 6 comprises a DEMUX (wavelength demultiplexer) 61, VOAs (Variable Optical Attenuators) 62-1 to 62-n, and a MUX (wavelength multiplexer) 63.
The DEMUX 61 receives a WDM (Wavelength Division Multiplexing) signal having n wavelengths multiplexed, and demultiplexes the received WDM signal into respective wavelengths. The VOAs 62-1 to 62-n have their attenuation amounts variably set for the respective wavelengths. The MUX 63 multiplexes the wavelength signals whose levels have been varied, to generate a WDM signal, and outputs the generated signal.
A fixed compensation device has a loss characteristic (loss profile) nearly uniform throughout the wavelength band of the signal beam. FIG. 20 illustrates such a loss characteristic of a fixed compensation device, wherein the vertical axis indicates insertion loss (IL) (dB) and the horizontal axis indicates wavelength (nm).
As seen from the graph, the loss is nearly uniform with respect to every wavelength in the wavelength band of the signal beam, exhibiting a nearly flat loss characteristic (in practice, some ripples are contained, but in the schematic illustration of FIG. 20, flatness of the loss characteristic is indicated simply by a straight line).
In a system configuration wherein a fixed compensation device having such a loss characteristic is inserted in the optical fiber transmission line connecting between the post-amplifier and the pre-amplifier, no particular inconvenience is caused if the aforementioned ASE setup for the pre-amplifier is executed.
Many of variable compensation devices, on the other hand, have loss characteristics involving periodicity in the direction of wavelength. FIG. 21 illustrates such a loss characteristic of a variable compensation device, wherein the vertical axis indicates insertion loss (IL) (dB), and the horizontal axis indicates wavelength (nm).
The loss characteristic is not uniform within the wavelength band of the signal beam, as depicted in FIG. 21, and because of this, although the loss is small at each wavelength peak of the signal beam, the loss increases if the wavelength is shifted even slightly from the wavelength peak. For example, at wavelengths λ1 and λ2 of the signal beam, the loss is approximately 3 dB, but at a wavelength λa shifted from the wavelengths λ1 and λ2, the loss increases to 8 dB.
In a system configuration wherein a variable compensation device having such a loss characteristic is inserted in the optical fiber transmission line connecting between the post-amplifier and the pre-amplifier, when the aforementioned ASE light is emitted to set up the pre-amplifier, the power of the ASE light greatly lowers due to the loss characteristic of the variable compensation device. At the time of the ASE setup, therefore, the gain of the pre-amplifier is determined using the ASE light whose loss is larger than an actual loss of the signal beam caused during the transmission.
This means, however, that the gain is determined to provide a target output value by using power which is lower than the receive power of an actual single-wavelength signal beam. Consequently, a gain excessively large for the actual signal beam is set, giving rise to a problem that the optical amplifier fails to be set up with accuracy.
FIG. 22 schematically illustrates the ASE setup of a system having a variable compensation device incorporated therein. An optical transmission system 5-1 comprises a transmitting station 51 and a receiving station 52-1 interconnected by an optical fiber transmission line f. The transmitting station 51 includes a post-amplifier 51a, and the receiving station 52-1 includes a TDC (Tunable Dispersion Compensator) 52a-1 and a pre-amplifier 52b. 
When performing the ASE setup for the pre-amplifier 52b in the optical transmission system 5-1, the post-amplifier 51a is caused to emit ASE light with transmit power corresponding to that of a single-wavelength signal of the operational signal beam. The ASE light is propagated through the optical fiber transmission line f and reaches the receiving station 52-1. After passing through the TDC 52a-1, the ASE light enters the pre-amplifier 52b. 
FIG. 22 also illustrates the power levels of the ASE light at respective points p1 to p3 (the vertical axes indicate optical power and the horizontal axes indicate wavelength). Specifically, the point p1 indicates the transmit power of the ASE light, and the point p2 indicates the receive power of the ASE light at the input end of the pre-amplifier 52b. The point p3 indicates the output power of the ASE light amplified by the pre-amplifier 52b. 
The TDC 52a-1 has a periodic loss characteristic as illustrated in FIG. 21. Thus, as the ASE light passes through the TDC 52a-1 having such a loss characteristic, the power of the ASE light lowers in such a manner that the optical power profile is trimmed off along the loss characteristic of the TDC 52a-1 (the wavelength regions other than the signal wavelengths within the band of the ASE light are trimmed off) (point p2).
At the receiving station 52-1, the gain of the pre-amplifier 52b is determined by using the ASE light passed through the TDC 52a-1 so that the output power of the pre-amplifier 52b may be equal to the target output value for an actual signal beam (target output value for a single-wavelength signal).
FIG. 23 illustrates the signal beam power observed during the operation of the system after the ASE setup, or more specifically, the power levels of an actual signal beam observed at the respective points p1 to p3 after the execution of the ASE setup illustrated in FIG. 22. Using the gain determined at the time of the ASE setup, the pre-amplifier 52b amplifies the actual signal beam.
However, the gain of the pre-amplifier 52b has been determined using the ASE light whose power was greatly reduced because of the passage through the TDC 52a-1 (ASE power after passage through TDC<signal beam power after passage through TDC), and this means that the gain has been determined to provide the target output value, on the basis of the level lower than the receive level of an actual single-wavelength signal beam. Accordingly, the set gain is too high for the actual signal beam, with the result that during the operation, a signal beam with excessively high power is output, as indicated by the point p3.